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Abstract:

A driver apparatus for an electroluminescent display comprising a
plurality of rows to be scanned and a plurality of columns which
intersect the rows to form a plurality of pixels, comprises addressable
row drivers, each row driver applying an output voltage to its associated
row when addressed. The value of the output voltage is approximately
equal to the numerical average of the threshold voltage for the
electroluminescent display and the voltage required to provide the
maximum desired pixel luminance for the electroluminescent display.
Bipolar column drivers each supply an output voltage to its associated
column. The output voltage is either positive or negative depending on
the desired luminance of the pixels. The range of both positive and
negative column output voltages is from zero volts to about one half of
the difference between the threshold voltage and the voltage to provide
the desired maximum pixel luminance for the electroluminescent display.

Claims:

1. A driver apparatus for an electroluminescent display comprising a
plurality of rows to be scanned and a plurality of columns which
intersect said rows to form a plurality of pixels, said driver apparatus
comprising: an addressable row driver applying an output voltage to a
respective row of said electroluminescent display, for a positive frame
the value of said output voltage being greater than a positive threshold
voltage for the electroluminescent display and less than a positive
voltage required for maximum electroluminescent display luminance and for
a negative frame the value of said output voltage being less than a
negative threshold voltage for the electroluminescent display and greater
than a negative voltage required for maximum electroluminescent display
luminance; and a bipolar column driver supplying an output voltage to the
column intersecting said respective row, for a given positive or negative
frame the output voltage being either a positive or negative voltage ramp
depending on the desired luminance of the pixel defined by the respective
row and intersecting column and the value of the output voltage applied
to the respective row.

2. A driver apparatus according to claim 1 wherein the shapes of the
positive and negative voltage ramps differ.

3. A driver apparatus according to claim 2 wherein the positive and
negative voltage ramps are non-linear.

4. A driver apparatus according to claim 1 wherein the ends of the
voltage ramps are timed such that the voltage ramps are supplied
substantially over the duration of each frame.

5. A driver apparatus according to claim 1 further comprising a sensor
generating a signal proportional to the electroluminescent display
luminance for a particular driver output voltage, said signal being used
to adjust the shape of the voltage ramps.

6. A driver apparatus according to claim 5 wherein said sensor is a
calibration pixel on said electroluminescent display.

7. A driver apparatus for an electroluminescent display comprising a
plurality of rows to be scanned and a plurality of columns which
intersect said rows to form a plurality of pixels, said driver apparatus
comprising: addressable row drivers, during a frame each row driver
applying an output voltage to its associated row when addressed, the
value of the output voltage being approximately equal to the numerical
average of the threshold voltage for the electroluminescent display and
the voltage required to provide the maximum desired pixel luminance for
the electroluminescent display; and bipolar column drivers, during the
frame each column driver supplying an output voltage to its associated
column, the output voltage being either a positive or negative voltage
ramp depending on the desired luminance of the pixel intersecting the
associated column that is being addressed, wherein the range of both
positive and negative voltage ramps is from zero volts to about one half
of the difference between the threshold voltage and the voltage required
to provide the desired maximum pixel luminance for the electroluminescent
display.

8. A driver apparatus according to claim 7 wherein the shapes of the
positive and negative output voltage ramps differ.

9. A driver apparatus according to claim 8 wherein the positive and
negative voltage ramps are non-linear.

10. A driver apparatus according to claim 8 further comprising a sensor
generating a signal proportional to the electroluminescent display
luminance for a particular driver output voltage, said signal being used
to adjust its shape of the voltage ramps.

11. A driver apparatus according to claim 10 wherein said sensor is a
calibration pixel on said electroluminescent display.

12. An electroluminescent display comprising: a plurality of rows to be
scanned; a plurality of columns which intersect said rows to form a
plurality of pixels; addressable row drivers, during a frame each row
driver applying an output voltage to its associated row when addressed;
and bipolar column drivers, during the frame each column driver supplying
an output voltage to its associated column, wherein during row addressing
over the frame the output voltage of each column driver is split into
positive and negative voltage ramps and the row output voltage is
adjusted commensurately, so that the threshold value of the
electroluminescent display is the difference between the absolute value
of the row output voltage and the maximum absolute value of the negative
column output voltage and so that the voltage for maximum pixel luminance
is the sum of the absolute value of the row output voltage and the
absolute value of the column output voltage.

13. An electroluminescent display according to claim 12 wherein the
shapes of the positive and negative voltage ramps differ.

14. An electroluminescent display according to claim 13 herein the
positive and negative voltage ramps are non-linear.

15. An electroluminescent display according to claim 12 further
comprising a sensor generating a signal proportional to the
electroluminescent display luminance for a particular driver output
voltage, said signal being used to adjust its shape of the voltage ramps.

16. An electroluminescent display according to claim 15 wherein said
sensor is a calibration pixel on said electroluminescent display.

17. A driver apparatus for an electroluminescent display comprising a
plurality of rows to be scanned and a plurality of columns which
intersect said rows to form a plurality of pixels, said driver apparatus
comprising: addressable row drivers, during a frame each row driver
applying an output voltage to its associated row, the value of the output
voltage corresponding to a gray level near the middle level for
electroluminescent display illumination; and bipolar column drivers
having a voltage modulation gray scale capability, during the frame, each
column driver supplying an output voltage to the pixels on an addressed
row, the output voltage being either a positive or negative voltage ramp
depending on the desired gray level of the pixels, the negative voltage
ramp ranging from zero volts to the difference between the threshold
voltage of the electroluminescent display and the voltage corresponding
to a gray level near the middle level for electroluminescent display
illumination and the positive voltage ramp ranging from zero volts to the
difference between the voltage corresponding to the highest (brightest)
gray level and the voltage corresponding to the gray level near the
middle level for electroluminescent display illumination.

18. A driver apparatus according to claim 17 wherein the ends of the
voltage ramps are timed such that the voltage ramps are supplied
substantially over the duration of the frame.

19. A bipolar column driver output stage to drive a column of an
electroluminescent display, said bipolar column driver output stage
comprising: positive and negative ramp control circuits receiving gray
scale information and being responsive to frame polarity input so that
only one of said ramp control circuits is enabled during a frame, said
ramp control circuits also receiving end point and voltage ramp signals,
each ramp control circuit being configured to output one of a positive
and negative ramp voltage ramp, wherein during the frame the enabled ramp
control circuit is conditioned to output either the positive voltage ramp
or the negative voltage ramp depending on the desired illumination of an
addressed pixel intersecting said column and a row voltage applied to the
addressed pixel; a charge store coupled to the ramp control circuits and
receiving the positive or negative voltage ramp output by the enabled
ramp control circuit; and an output buffer responsive to said charge
store to modulate a voltage supply thereby to generate output column
voltage pulses.

[0002] The present invention relates to an electroluminescent display
using bipolar column drivers.

BACKGROUND

[0003] Electroluminescent displays are advantageous by virtue of their low
operating voltage with respect to cathode ray tubes, their superior image
quality, wide viewing angle and fast response time over liquid crystal
displays, and their superior gray scale capability and thinner profile as
compared to plasma display panels.

[0004] As shown in FIGS. 1 and 2, an electroluminescent display has two
intersecting sets of parallel, electrically conductive address lines
called rows (ROW 1, ROW 2, etc.) and columns (COL 1, COL 2, etc.) that
are disposed on either side of a phosphor film encapsulated between two
dielectric films. A pixel is defined as the intersection point between a
row and a column. Thus, FIG. 2 is a cross-sectional view through the
pixel at the intersection of row ROW 4 and column COL 4, in FIG. 1. Each
pixel is illuminated by the application of a voltage across the
intersection of the row and column defining the pixel using row and
column drivers (not shown) coupled to the rows and columns.

[0005] Matrix addressing entails applying a voltage below the threshold
voltage to a row while simultaneously applying a modulation voltage of
the opposite polarity to each column that bisects that row. The voltages
on the row and the columns are summed to give a total voltage in
accordance with the illumination desired on respective sub-pixels,
thereby generating one line of the image. An alternate scheme is to apply
the maximum sub-pixel voltage to the row and apply a modulation voltage
of the same polarity to the columns that intersect that row. The
magnitude of the modulation voltage is up to the difference between the
maximum voltage and the threshold voltage to set the pixel voltages in
accordance with the desired image. In either case, once each row is
addressed, another row is addressed in a similar manner until all of the
rows have been addressed. Rows that are not addressed are left at open
circuit. The sequential addressing of all rows constitutes a complete
frame. Typically, a new frame is addressed at least about fifty (50)
times per second to generate what appears to the human eye as a
flicker-free video image.

[0006] In order to generate realistic video images with flat panel
displays, it is important to provide the required luminosity ratios
between gray levels where the driving voltage is regulated to facilitate
gray scale control. This is particularly true for electroluminescent
displays where gray scale control is exercised through control of the
output voltage on the column drivers for the display.

[0007] Traditional thin film electroluminescent displays employing thin
dielectric layers that sandwich a phosphor film between driving
electrodes is not amenable to gray scale control through modulation of
the column voltage, due to the very abrupt and non-linear nature of the
luminance turn-on as the driving voltage is increased. By way of
contrast, electroluminescent displays employing thick, high dielectric,
constant dielectric layered pixels have a nearly linear dependence on the
luminance above the threshold voltage, and are thus more amenable to gray
scale control by voltage modulation. However, even in this case if the
gray scale voltage levels are generated by equally spaced voltage levels
then the luminance values of the gray levels are not in the correct
ratios for video applications.

[0008] The gray level information in a video signal is digitally encoded
as an 8-bit number or code. These digital gray level codes are used to
generate reference voltage levels Vg that facilitate the generation
of luminance levels (Lg) for each gray level in accordance with an
empirical relationship of the form:

Lg=f(Vg)=An.sup.γ (Equation 1)

[0009] where:

[0010] A is a constant;

[0011] n is the gray level code; and

[0012] γ is typically between 2 and 2.5.

[0013] An electroluminescent display driver with gray scale capability
resembles a digital-to analog (D/A) device with an output buffer. The
purpose is to convert an incoming 8-bit gray level code from the video
source to an analog output voltage for electroluminescent display
driving. There are various types of gray scale drivers employing
different methods of performing the necessary digital-to-analog
conversion. A preferred type and method uses a linear ramping voltage as
a means of performing the D/A conversion. For this type of gray scale
driver, the digital gray level code is first converted to a pulse-width
through a counter operated by a fixed frequency clock. The time duration
of the pulse-width is a representation of, and corresponds to, the
digital gray level code. The pulse-width output of the counter in turn
controls the turn-on of a capacitor sample-and-hold circuit which
operates in conjunction with an externally generated linear voltage ramp
to achieve the pulse-width to voltage conversion. Since the voltage ramp
has a linear relationship between the output voltage and time, the
pulse-width representation of the digital gray level code results in a
linear gray level voltage at the driver output. The luminance created for
each gray level is thus dependent on the relationship between the voltage
applied to a pixel and the pixel luminance, which is dependent on the
electro-optical characteristic of the electroluminescent display. This
luminance-voltage characteristic is normally different from the ideal
characteristic, and therefore Gamma correction is necessary.

[0014] The relationship between the voltage applied to a pixel and its
luminance is typified by the curve in FIG. 3. To achieve proper color
balance for the electroluminescent display, a Gamma correction is made to
the linear voltage ramp to achieve the relationship between luminance and
a gray level given by Equation 1. For the luminance versus voltage curve
of FIG. 3, the linear voltage ramp is replaced by the non-linear voltage
ramps shown in FIG. 4. The non-linear voltage ramps can be generated
using analogue circuitry such as that taught in co-pending U.S. patent
application Publication No. 2004/0090402 to Cheng or by other means as
may be known in the art. The non-linear voltage ramps are different for
positive and negative row voltages because in the former case the pixel
voltage is the difference between the row and column voltages and in the
latter case the pixel voltage is the sum of the row and column voltages.
The luminance begins to rise above the threshold voltage in a non-linear
fashion for the first few volts above the threshold voltage, and then
rises in an approximate linear fashion before saturating at a fixed
luminance. The portion of the curve used for electroluminescent display
operation is the initially rising portion and the linear portion. The
effects of differential loading of the driver outputs complicate the
relationship. To negate the effect of variable loading and to improve the
energy efficiency of the electroluminescent display, a driver employing a
sinusoidal drive voltage with a resonant energy recovery feature is
typically employed. Such a driver is disclosed in U.S. Pat. No. 6,448,950
to Cheng and U.S. patent application Publication No. 2003/0117421 to
Cheng, the contents of which are incorporated herein by reference. U.S.
patent application Publication No. 2004/0090402 to Cheng teaches a method
and apparatus to realize the necessary Gamma correction of an
electroluminescent display panel conveniently at the D/A conversion stage
by replacing the normal linear voltage ramp with a special
`double-inverted-S` non-linear voltage ramp. The use of this non-linear
voltage ramp enables adjustment of the voltages for the gray levels to
generate a gray scale response similar to that described by the empirical
relationship given by Equation 1.

[0015] As described in U.S. Pat. No. 6,448,950 to Cheng, a major portion
of the power consumed by passively addressed electroluminescent displays
is fed through the column drivers due to a parasitic capacitive coupling
between the columns and the non-addressed rows. This patent teaches a
means to reduce this power consumption by providing a sinusoidal driving
waveform to minimize peak current and to recover a major portion of the
energy through a resonant energy recovery circuit. Co-pending U.S.
Provisional Patent Application No. 60/646,326 filed on Feb. 23, 2005
teaches a means to increase further the energy efficiency by ensuring
that as much of the energy from the electroluminescent display panel is
recovered by the energy recovery circuit and not dissipated in parallel
parasitic current loops through ground and through the supply voltage
lines for the drivers. Although, these measures provide for energy
recovery, they do not reduce the current flow through the drivers to
zero. As will be appreciated, improvements in electroluminescent display
energy efficiency and cost reductions in the column drivers may also be
realized if the current flowing from the output of the column drivers can
be reduced.

[0016] Other techniques for driving electroluminescent displays have been
considered. For example, U.S. Pat. No. 6,636,206 to Yatabe discloses a
system and method of driving a display device so as to display a gray
scale image without causing a significant increase in power consumption.
Pixels disposed at locations corresponding to respective intersections of
a plurality of scanning lines extending along rows and a plurality of
data lines extending along columns are driven. A single scanning line is
selected during one horizontal scanning period and a selection voltage is
applied to the scanning line for one half of the scanning period. Another
adjacent scanning line is selected during the next horizontal scanning
period and the selection voltage is applied to the scanning line for the
other half of the scanning period. At the same time, a turn-on and
turn-off voltage is applied to a pixel at a location corresponding to the
selected scanning line such that the turn-on voltage is applied for a
length corresponding to a gray level in the period during which the
selection voltage is applied. The turn-off voltage is applied during the
remaining period.

[0017] U.S. Pat. No. 5,315,311 to Honkala discloses a method and apparatus
for reducing power consumption in an AC-excited electroluminescent
display. Each row of the display matrix is alternatively driven by
positive and negative row drive pulses. The magnitudes of successive row
drive pulses are different. Each column of the display matrix is driven
individually by modulation voltage pulses synchronized to the row
addressing sequence. The modulation voltage pulses have a maximum
amplitude and an "on"-state polarity equal to that of the
larger-magnitude row drive pulse.

[0018] U.S. Pat. No. 6,803,890 to Velayudehan et al. discloses a system
and method for addressing and achieving gray scale in an
electroluminescent display using a waveform having at least one positive
ramped modulating pulse and zero or more non-ramped modulating pulses.
The pulses are applied to the electroluminescent display successively to
form a scan pulse that is applied across an electrode row and electrode
column.

[0019] Although various techniques for driving electroluminescent displays
exist, improvements are continually being sought. It is therefore an
object of the present invention to provide a novel electroluminescent
display using bipolar column drivers.

SUMMARY

[0020] The electroluminescent display driving method and apparatus enables
a reduction in the output current of column drivers by splitting the
required column voltage into positive and negative portions and adjusting
the row voltage commensurately, so that the display threshold voltage is
determined as being the difference between the absolute value of the row
voltage and the maximum absolute value of the negative column voltage,
and so that the voltage for maximum luminance is the sum of the absolute
value of the row voltage and the absolute value of the column voltage.

[0021] In one embodiment, the rows of the electroluminescent display are
addressed sequentially, and the columns bisecting an addressed row are
simultaneously addressed. Column drivers provide a bipolar voltage output
so that the threshold voltage for the electroluminescent display pixels,
defined as the voltage for the onset of light emission, is equal to the
difference between the absolute value of the row voltage and the maximum
absolute value of the voltage from the positive output of the column
drivers and further so that the voltage for maximum luminance of an
electroluminescent display pixel is equal to the sum of the absolute
value of the row voltage and the maximum absolute value of the voltage
from the negative output of the column drivers.

[0022] In this embodiment, the voltage of the addressed row may be
alternately positive and negative with respect to a common reference
voltage, which may be ground. The electroluminescent display may also be
provided with gray scale capability wherein the number of gray levels are
divided between the positive and negative outputs of the column drivers.
The division is made on the basis that the gray level selection
probability in typical video applications reaches a peak in mid-range
gray levels. As a result, a gray level near the most commonly selected
gray level is chosen to correspond to a zero column voltage. This results
in about one half of the gray levels corresponding to a negative column
voltage and about one half of the gray levels corresponding to a positive
column voltage. It will be appreciated that this division can of course
be adjusted based on a detailed analysis of typical gray level
distribution for video.

[0023] The gray levels may be generated using a voltage ramp where the end
of the voltage ramp, which defines the voltage level for each of the gray
levels assigned to each of the positive and negative outputs of the
column drivers, is timed such that the times for the end of the voltage
ramp for these gray levels are spaced substantially over the entire
duration of the period during which a row is addressed. The voltage ramp
used to define the gray levels may be non-linear with respect to time to
account for the relationship between display luminance and the driving
voltage. Alternatively, a tailored non-linear relationship between the
voltage at the end of the voltage ramp and the gray levels can be
realized by employing a non-linear voltage ramp and a variable frequency
clock using a voltage controlled oscillator to vary the clock frequency
over the duration of the voltage ramp. The shape of the voltage ramp
curve with respect to time or the frequency of the voltage controlled
oscillator is adjusted in accordance with a sensor incorporated into the
electroluminescent display that generates a signal proportional to the
luminance for a particular driving voltage and by providing feedback to
the voltage ramp generator or the voltage controlled oscillator to vary
the clock frequency in accordance with the required gray levels.

[0024] In one form, the sensor comprises an extra calibration pixel
fabricated on the electroluminescent display substrate outside of the
video portion of the electroluminescent display. The extra calibration
pixel has the same operational and aging characteristics as the
electroluminescent display pixels. A photo-diode or similar light
measuring device is mounted on the rear of the electroluminescent display
substrate immediately behind the extra calibration pixel or in proximity
to the extra calibration pixel so that it measures light transmitted
through the electroluminescent display substrate that is proportional to
the luminance of the extra calibration pixel.

[0025] Accordingly, in one aspect there is provided a driver apparatus for
an electroluminescent display comprising a plurality of rows to be
scanned and a plurality of columns which intersect said rows to form a
plurality of pixels, said driver apparatus comprising:

[0026] addressable row drivers applying an output voltage to the rows, the
value of which is greater than the threshold voltage for the
electroluminescent display and less than that required to provide the
maximum desired luminance for a pixel; and

[0027] bipolar column drivers supplying an output voltage to the columns,
the output voltage being either a positive or negative voltage depending
on the desired luminance of the pixels.

[0028] According to another aspect there is provided a driver apparatus
for an electroluminescent display comprising a plurality of rows to be
scanned and a plurality of columns which intersect said rows to form a
plurality of pixels, said driver apparatus comprising:

[0029] addressable row drivers, each row driver applying an output voltage
to its associated row when addressed, the value of which is approximately
equal to the numerical average of the threshold voltage for the
electroluminescent display and the voltage required to provide the
maximum desired pixel luminance for the electroluminescent display; and

[0030] bipolar column drivers, each supplying an output voltage to its
associated column, the output voltage being either positive or negative
depending on the desired luminance of the pixels, wherein the range of
both positive and negative column output voltages is from zero volts to
about one half of the difference between the threshold voltage and the
voltage required to provide the desired maximum pixel luminance for the
electroluminescent display.

[0031] According to yet another aspect there is provided an
electroluminescent display comprising:

[0032] a plurality of rows to be scanned;

[0033] a plurality of columns which intersect said rows to form a
plurality of pixels;

[0034] addressable row drivers each applying an output voltage to its
associated row when addressed; and

[0035] bipolar column drivers each supplying an output voltage to its
associated column, wherein during row addressing the output voltage of
each column driver is split into positive and negative portions and the
row voltage is adjusted commensurately, so that the electroluminescent
display threshold voltage is the difference between the absolute value of
the row voltage and the maximum absolute value of the negative column
voltage and so that the voltage for maximum pixel luminance is the sum of
the absolute value of the row voltage and the absolute value of the
column voltage.

[0036] According to yet another aspect there is provided a driver
apparatus for an electroluminescent display comprising a plurality of
rows to be scanned and a plurality of columns which intersect said rows
to form a plurality of pixels, said driver apparatus comprising:

[0037] addressable row drivers each applying an output voltage to its
associated row, the value of which corresponds to a gray level near the
middle level for an electroluminescent display pixel; and

[0038] bipolar column drivers having a voltage modulation type gray scale
capability, the column drivers supplying an output voltage to the pixels
on an addressed row, the output voltage being either positive or negative
depending on the desired gray level of the pixels, the range of column
voltage when negative being from zero volts to the difference between the
threshold voltage and the voltage corresponding to a gray level near the
middle level for the electroluminescent display pixel and when positive
being from zero volts to the difference between the voltage corresponding
to the highest (brightest) gray level and the voltage corresponding to
the gray level near the middle level for the electroluminescent display
pixel.

[0039] According to yet another aspect there is provided a bipolar column
driver output stage comprising:

[0040] positive and negative ramp control circuits receiving gray scale
information and being responsive to frame polarity input so that only one
of said ramp control circuits is enabled at a time, said ramp control
circuits also receiving end point and voltage ramp signals;

[0041] a charge store coupled to the ramp control circuits and receiving
the voltage ramp output by the enabled ramp control circuit; and

[0043] According to still yet another aspect there is provided a method of
driving a row of pixels of an electroluminescent display comprising a
plurality of rows and a plurality of pixels intersecting said rows to
define a plurality of pixels, said method comprising:

[0044] addressing the pixel row by applying an output voltage thereto; and

[0045] applying either a positive or a negative voltage to the columns
intersecting the addressed row depending on the desired gray level of the
pixels in the addressed row.

[0046] The electroluminescent display drivers provide for improved energy
efficiency for video applications and for improved gray scale control by
modulation of the voltage applied to the column electrodes using a
non-linear or step-wise linear voltage ramp.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Embodiments will now be described more fully with reference to the
accompanying drawings, in which:

[0048]FIG. 1 is a plan view of a typical arrangement of rows and columns
of pixels forming part of an electroluminescent display;

[0049]FIG. 2 is a cross-section through a single pixel of the
electroluminescent display of FIG. 1;

[0050]FIG. 3 is a luminance versus applied voltage curve for the
electroluminescent display of FIG. 1;

[0051]FIG. 4 shows voltage ramp curves applied to the output of unipolar
column drivers during the application of a negative row voltage and
during the application of a positive row voltage to generate gray scale
luminance from the luminance versus voltage curve of FIG. 3;

[0053] FIG. 6 shows voltage ramp curves applied to a positive output and
to a negative output of the bipolar column driver output stage of FIG. 5
during the application of a negative row voltage pulse to generate the
same gray scale luminance as the unipolar column drivers referenced with
respect to FIG. 4; and

[0054] FIG. 7 shows voltage ramp curves applied to the positive output and
to the negative output of the bipolar column driver output stage of FIG.
5 during the application of a positive row voltage pulse to generate the
same gray scale luminance as the unipolar column drivers referenced with
respect to FIG. 4.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0055] To improve the efficiency of electroluminescent displays of the
type such as that shown in FIG. 1, bipolar column driver output stages or
simply bipolar column drivers are used to drive the column electrodes or
address lines during matrix addressing. The use of bipolar column drivers
reduces the power consumption of the electroluminescent display and
reduces the current flow in the column drivers by reducing the maximum
voltage that must be output from the column drivers.

[0056] In one embodiment, the electroluminescent display employs row
drivers that set the row voltage to a value that is between the threshold
voltage for the electroluminescent display and the voltage required for
maximum display luminance. Bipolar column drivers with voltage modulation
gray scale capability are employed. The bipolar column drivers set the
column voltage to a positive or negative value, depending on whether the
required gray level for the electroluminescent display pixel defined by
the intersection of that column and the addressed row is greater than or
less than the gray level when the electroluminescent display pixel
voltage is equal to the row voltage. The bipolar column drivers differ
from those of the prior art in that they have a bipolar output. The
bipolar column drivers may also have a substantially different voltage
ramp for the negative polarity output than they do for the positive
polarity output to accommodate the non-linear nature of gray levels. On
the assumption that the row and column voltages are measured with respect
to ground or a common reference voltage and if the row voltage is
positive, then the lowest gray level corresponds to the highest positive
voltage output from a bipolar column driver and the highest gray level
corresponds to the lowest negative output voltage from the bipolar column
driver. The polarity of the row voltage may be alternated from frame to
frame to minimize the average applied voltage to the row for minimization
of electroluminescent display degradation due to electric field assisted
diffusion of atomic species in the electroluminescent display structure.
The column voltages therefore may also be correspondingly alternated from
frame to frame. Separate voltage ramp generating circuits can be employed
for positive and negative column output voltages to achieve the required
gray scale fidelity. The voltage ramp used to define the gray levels may
be non-linear with respect to time to account for the relationship
between display luminance and the driving voltage. Alternatively, a
tailored non-linear relationship between the voltage at the end of the
voltage ramp and the gray levels can be realized by employing a
non-linear voltage ramp and a variable frequency clock using a voltage
controlled oscillator to vary the clock frequency over the duration of
the voltage ramp. The shape of the voltage ramp curve with respect to
time or the frequency of the voltage controlled oscillator is adjusted in
accordance with a sensor incorporated into the electroluminescent display
that generates a signal proportional to the luminance for a particular
driving voltage and by providing feedback to the voltage ramp generator
or the voltage controlled oscillator to vary the clock frequency in
accordance with the required gray levels.

[0057] The sensor may comprise an extra calibration pixel fabricated on
the electroluminescent display substrate outside of the video portion of
the electroluminescent display. The extra calibration pixel has the same
operational and aging characteristics as the electroluminescent display
pixels. A photo-diode or similar light measuring device is mounted on the
rear of the electroluminescent display substrate immediately behind the
extra calibration pixel or in proximity to the extra calibration pixel so
that it measures light transmitted through the electroluminescent display
substrate that is proportional to the luminance of the extra calibration
pixel.

[0058] FIG. 5 illustrates one of the bipolar column drivers. As can be
seen, video data with gray scale information is provided as input to a
digital comparator circuit 100. The output from the comparator circuit
100 is input into two ramp control circuits 102 and 104, one for negative
row voltage pulses and the other for positive row voltage pulses. To
determine the end point for positive and negative column driver output
voltage ramps, Vramp+/+ and Vramp+/-. inputs are provided to the ramp
control circuit 104 for the positive row voltage pulses. For positive and
negative column driver output voltage ramps, Vramp-+. and Vramp-/- inputs
are provided to the ramp control circuit 102 for the negative row voltage
pulses. A frame polarity signal is input to the ramp control circuits 102
and 104 to select the active ramp control circuit. The output voltage
ramps from the ramp control circuits 102 and 104 charge a hold capacitor
108 so that the desired gray level voltages determined on the basis of
the input video data are input to an output buffer circuit 110, which
modulates column voltage supplies Vpp+ and Vpp- to provide voltage pulses
with the correct amplitude and polarity at a suitably low output
impedance, to the column electrodes thereby to drive the
electroluminescent display columns.

[0059] The use of bipolar column drivers reduces power consumption of the
electroluminescent display for video applications since on average, the
column voltage to generate the statistical distribution of gray levels
typical of a video image is for a large fraction of the time close to
half of the column voltage for maximum luminance. The power delivered
through the columns is much greater than the power delivered through the
rows, since the rows are addressed sequentially, with the non-addressed
rows remaining at open circuit during electroluminescent display
operation so that only the pixels on the addressed row are charged,
whereas the columns are addressed simultaneously while a selected row is
addressed, causing partial charging of all of the non-addressed rows as
well as the addressed row due to capacitive coupling of the columns
through the intersecting rows. This parasitic power drain to the
non-addressed rows is greatest when half of the column outputs are at or
near zero volts and the other half are at or near their maximum voltage.

[0060] The bipolar column drivers reduce this parasitic drain by setting
the row voltage near the most frequently set voltages for the pixels so
that the column voltages will be on average closer to zero.

[0061] The use of bipolar column drivers also enables the possibility of
using a smaller silicon die for the column drivers with a defined number
of channels since the total voltage ramp range is reduced. In large
format high resolution displays such as those for high definition
television, the voltage ramp rate must be sufficiently fast to allow the
required gray level voltage to be reached during the time allowed for
addressing each row. This together with the display capacitance
determines the required output current for the column drivers so that the
required voltage ramp rate is achieved. The required current in turn
establishes the required silicon area for FET based column drivers to
allow construction of a gate of sufficient width to minimize I2R
losses and thus, inhibit excessive heat generation in the column drivers.
Since the electroluminescent display represents a capacitive load on the
column drivers, the output current from the column drivers is
proportional to the rate of change of voltage in the gray scale
generating ramp. Thus the rate of change in voltage, dV/dt, is
proportional to the maximum voltage that a particular column driver
output can be called upon to deliver, and inversely proportional to the
time available to ramp the voltage to this level. The use of bipolar
column drivers also reduces the maximum output current that can be
demanded by reducing the maximum voltage that may be required. By
adjusting the clock that determines the end-point for the voltage ramp
for a particular gray level so that the highest gray level for each of
the positive output and negative output column drivers is reached only at
or near the maximum amount of time available to address each row, dV/dt
can be reduced with respect to that for an electroluminescent display
using unipolar column drivers in proportion to the reduction in maximum
positive or negative voltage demanded from the column driver in question.

[0062] Embodiments are illustrated by the following examples, which are
not intended to be limiting, but merely to provide illustrations of
certain useful embodiments.

Example 1

[0063] This example illustrates a particular embodiment where the required
maximum negative and positive output voltages for the column drivers are
nearly equal, and where the voltage versus luminance curve is non-linear.
In this case, there will be a significantly larger number of gray levels
provided by one polarity of output from the column drivers than from the
other. The gray levels are generated by terminating a linear voltage ramp
in the column driver output using a digital clock with equally spaced
gray level codes. If 20% of the gray levels for the electroluminescent
display are provided by one polarity and 80% by the other polarity, then,
relative to the requirements for a similar display employing unipolar
column drivers, the spacing between gray level codes for the polarity
providing 20% of the gray levels can be increased by up to a factor of
five (5) and the spacing between gray level codes for the other polarity
can be increased by up to 25%. If this is done and with the assumption
that the maximum voltage for each of the positive output and negative
outputs of the column drivers is 50% of that for the column drivers for a
similar display operated using unipolar column drivers, dV/dt and hence
the maximum current demand for the bipolar column drivers is only 50% of
that for unipolar column drivers. Since the maximum power dissipation is
proportional to I2R, the corresponding reduced instantaneous power
level is 25% of that for unipolar column drivers driving a similar
display for both positive and negative outputs of the bipolar column
drivers.

[0064] The required silicon area for the bipolar column drivers is
determined in part by the instantaneous power dissipation requirement and
in part by the average power dissipation requirement averages over a
frame, depending on the heat flow dynamics within the column driver chip
and the heat sinking efficiency for the column driver. However, the above
analysis shows by the maximum power dissipation, the reduction in the
maximum required power allows for a substantial reduction in the required
silicon area, and hence a significant reduction in the cost of the column
drivers, which represent a major portion of the cost of large format high
resolution displays.

Example 2

[0065] This example illustrates gray scale ramps for use with bipolar
column drivers to provide the necessary Gamma correction for a full color
display employing bipolar column drivers. The ramps are different for
positive and for negative applied row voltages, since in one case the
pixel voltage is the algebraic sum of the column and row voltages, and in
the other case the pixel voltage is the difference between the row and
column voltages. FIGS. 6 and 7 show how the required voltage ramps to
generate good color fidelity with unipolar column drivers as shown in
FIG. 4 can be adapted for use with bipolar column drivers. The horizontal
dotted line on FIG. 4 shows the division of the unipolar column driver
voltage range between the ranges for positive and negative voltage output
for the corresponding bipolar column driver. The two vertical dotted
lines on FIG. 4 show the corresponding division of digital clock counts
corresponding to gray levels for negative and for positive row voltage
pulses.

[0066] The solid curves in FIG. 6 show the direct transposition of the
unipolar voltage ramp of FIG. 4 for negative row voltage pulses for an
equivalent bipolar column driver. The dotted line shows a five (5) times
scaling of the digital clock counts for the voltage ramp for the negative
output, which has the smaller number of gray levels, so that the voltage
ramp extends over a greater fraction of the duration of a row pulse to
reduce dV/dt. For negative row voltage pulses, the positive bipolar
column driver output voltage Vb-/+(n) for the nth clock count
is given in terms of the unipolar column driver output voltage Vu-
as:

[0067] In a similar manner the solid curves in FIG. 7 show the direct
transposition of the unipolar voltage ramp of FIG. 4 for positive row
voltage pulses for an equivalent bipolar column driver. In this case the
negative bipolar column driver output voltage Vb+/-(n) for the
nth clock count is given in terms of the unipolar column driver
output voltage Vu+ for the nth clock count as:

[0069] Although preferred embodiments have been described, those of skill
in the art will appreciate that variations and modifications may be made
without departing from the spirit and scope thereof as defined by the
appended claims.